TS 320 
.S4 
Copy 1 



DEPARTMENT OF COMMERCE 



Scientific Papers 



OF THE 



Bureau of Standards 

S. W. STRATTON, Director 



No. 396 

THERMAL AND PHYSICAL CHANGES ACCOMPANY- 
ING THE HEATING OF HARDENED 
CARBON STEELS 

BY 

HOWARD SCOTT, Assistant Physicist 
H. GRETCHEN MOVIUS, Assistant Physicist 
Bureau of Standards 



SEPTEMBER 20, 1920 




PRICE, S CENTS 

Sold only by the Superintendent of Documents, Government Printing Office,', 
Washington, D. C. 



Washington 
government printing office 

1920 



DEPARTMENT OF COMMERCE 



Scientific Papers 



OF THE 



Bureau of Standards 



S. W. STRATTON, Director 



No. 396 

thermal and physical changes accompany- 
ing the heating of hardened 
carbon steels 

BY 

HOWARD SCOTT, Assistant Physicist 

H. GRETCHEN MOVIUS, Assistant Physicist 

Bureau of Standards 



SEPTEMBER 20, 1920 




PRICE, S CENTS 

Sold only by the Superintendent of Documents, Government Printing Office, 
Washington, D. C. 



WASHINGTON 
GOVERNMENT PRINTING OFFICE 

1920 



tp 






Digitized by the Internet Archive 
in 2011 with fidrftfirftf from 

OCT 29 1929, 

The Library of Congress 




http://www.archive.orig/details/thermalphysicalcOOscot 



^ 



THERMAL AND PHYSICAL CHANGES ACCOMPANY- 
ING THE HEATING OF HARDENED CARBON STEELS 



By Howard Scott and H. Gretchen Movius 



CONTENTS Page 

I. Introduction 537 

II. Experimental method 539 

III. Heat evolution Ac, 540 

1 . Effect of rate of heating 544 

2. Effect of tempering temperature 545 

3 . Effect of time at tempering temperature 546 

4 . Effect of composition 548 

5. Effect of austenitic structure 549 

IV. Relation of changes in physical properties to heat evolution 551 

1 . Martensitic steel 551 

2. Austenitic steel 554 

V. Summary 555 

I. INTRODUCTION 

The widespread interest which has been recently expressed in 
the properties of steel in the "blue-heat" range and in the subject 
of "temper brittleness" makes it highly desirable to study in 
detail the transformations in steel below the Aj change. In a 
previous paper 1 the authors have pointed out certain thermal 
characteristics of the magnetic change in cementite as observed in 
annealed steels by means of thermal analysis. In this paper the 
subject under investigation is the thermal change observed in 
hardened steels on heating below Ac t . 

Outside of the possible bearing of such information on the low- 
temperature properties mentioned, there remains the desirability 
of establishing fundamental characteristics of steel. The one in 
question is of particular value in that it may furnish a practical 
basis for defining the natural boundary between martensite and 
the troostite of tempering, which from present information is very 
indefinite. 

A survey of the changes in some of the physical properties of 
carbon steels on tempering would, on account of certain incon- 
sistencies, lead one to doubt the existence of a sharp demarcation 
between the constituents — martensite and troostite. Heating 

1 Chemical and Metallurgical Engineering, 22, p. 1069; June 9, 1920. 

537 



538 Scientific Papers of the Bureau of Standards Ivoi. 16 

curves of hardened steels, however, have shown a well-marked 
heat evolution ending around 300 C. Such heat evolution would 
be expected from the usual conception of the formation of mar- 
tensite; namely, that one or more of the transformations occurring 
on slow cooling are suppressed by quenching. The consummation 
of the suppressed transformation (or transformations) is a mani- 
festation of the completion of the constitutional change and, 
therefore, evidence of a boundary between two constituents, pre- 
sumably martensite and troostite. Whether the end of this heat 
evolution should be used to define those constituents the future 
will decide ; the present work seeks only to establish its nature in a 
variety of carbon steels and its relation to accompanying changes 
in some of the physical properties. 

In the literature some work has appeared on this heat evolution 
in hardened carbon steels. Osmond 2 and Maurer 2 have given 
inverse-rate heating curves; Heyn and Bauer 3 and Portevin 3 
have given differential curves showing the phenomenon. 

The temperature values for the transformation are somewhat 
higher than those obtained here. In general, the curve inflections 
are neither prominent enough nor the statement of operating 
details sufficient to allow of a precise definition of the transforma- 
tion characteristics. Also the effect of important variables has 
not been determined. This phenomenon has been observed also 
by continuous measurement of the changes in some physical 
properties on heating. Grenet * detected an inflection in the ex- 
pansion and electric -resistance curves of a high-carbon steel, 
Chevenard 5 in expansion curves, and Honda 6 in magnetic- 
induction curves. The magnetic curves are the only ones which 
seem to follow closely the progress of the heat change. Brush 7 
has made extensive observations on the heat evolution at ordinary 
temperatures in recently hardened steels. He noted a heat evo- 
lution, greatest immediately after hardening, gradually diminishing 
in rate with time and becoming imperceptible after several weeks. 
The physical changes accompanying this spontaneous evolution 
were very small in comparison with those accompanying even 
slight tempering. 

While the present research is confined to carbon steels because 
of their fundamental importance, it is being extended to alloy 

2 Osmond, J. Iron and Steel Inst., p. 38; No. 1, 1890. Maurer, Rev. de Met., 5, p. 711; 1908. 

3 Heyn and Bauer, J. Iron and Steel Inst., 79, p. 109; 1909. Portevin, Rev. de Met.. 13, p. 9; 1916. 
* Grenet, Rev. de Met., 1, p. 353; 1904. 

6 Chevenard, Rev. de Met., 14, p. 610; 1917. 

a Honda, Sci. Reports Tohoku Imp. Univ., 6, p. 149; 1917. 

7 Brush, Bull. A. I. M. M. E. No. 153, p. 2389; 1919. 



Scot! "1 
MoTiusj 



Thermal Changes of Hardened Steels 



539 



steels in order to obtain further light on the effect and function of 
the alloying elements. 

II. EXPERIMENTAL METHOD 

The inverse-rate method of obtaining thermal curves has been 
used at the Bureau of Standards as the most effective and satis- 
factory method for studying the transformations in steel. Used 
in connection with the apparatus already described 8 excellent 
curves can be obtained at the low temperatures where the heat 
evolution under examination is found. The details of mounting, 
size of sample, and operation are given in the above reference. A 
temperature interval corresponding to 20 microvolts on a platinum, 
platinum-rhodium thermocouple was used in this investigation. 

TABLE 1. — Results of Chemical Analyses of Steels Investigated 



c 


Mn 


Si 


P 


S 


Per cent 
0.40 

.44 
a. 46 
a. 73 

.95 
1.01 
1.94 


Per cent 


Per cent 
0.01 
.02 
,06 
.01 
.24 
.01 
.01 


Per cent 


Per cent 


1.00 
.35 
.38 
.22 






0.02 
.04 
.02 


0.05 
.05 
.01 
.005 
.005 











a Furnished by courtesy of Carnegie Steel Co. 

The materials studied were seven steels of the compositions given 
in Table 1. Heating for quenching, as noted on the curves and 
in the tabulated results, was carried out on the prepared samples 
by introduction into an electrically heated alundum-tube furnace 
wound with resistance wire. Charcoal was present to reduce oxi- 
dation, and a platinum, platinum-rhodium thermocouple was used 
for the measurement of temperature. In tempering, the specimen 
was heated 30 minutes in an oil or nitrate bath, as required by the 
temperature. Temperatures below 300 C were measured with a 
mercury thermometer. 

All samples of the quenched 0.95 per cent C steel not receiving 
subsequent tempering were run within from 1 to 3 days after 
quenching. All the other steels not tempered were run within 
from 6 to 16 days after treatment, excepting the 1.01 per cent C 
and 1 .94 per cent C steels (curve j) , which were run the day follow- 
ing treatment. 

8 Scott and Freeman, Bull. A. I. M. M. E. No. 152, p. 1429; 1919. Also B. S. Sci. Papers, No. 348. 



54-0 Scientific Papers of the Bureau of Standards 

III. HEAT EVOLUTION Ac t 



{.Vol. 16 



The principal phenomenon under consideration here, the heat 
evolution on heating hardened carbon steels, will be designated as 
"Ac t ," a notation used by one of the authors • for the same phe- 
nomenon in a high-alloy steel. 



°C 
500 



WO 



300 



200 



/oo 



\ 0.75 Per cent C Stecf 

Quenched from 800 °C 



\ 



!© 



® 



© I© 



'}-£" 



■-E 



{-/V 



=;'-/V 



-At 



:■■-£ 



■:.-// 



K <3-c> 









\-J3 \ 






Time interval in seconds 

/O IS 10 IS IS 20 20 2S ¥5 SO SS 



FlG. I. — Inverse-rate heating curves of hardened steel, showing effect of rate of heating 

on Act 

The thermal curves, taken to show the effect of several variables 
on the transformation Ac t , are shown in Figs. 1, 2, 3, 4, and 5. 
For the reduction of the thermal-curve data to tabular form, the 



• Scott, Bull. A. I. M. M. E. No. 146, p. 157; Feb.. 1919. Also B. S. Sci. Papers, No. 335. 



Scett 1 
Meviusl 



Thermal Changes of Hardened Steels 



54i 









Off 5 Per cenf C Stee/ 








X 


Quenched 


/n 


<?// from 


eoo°c 






°c 

6)00 


1** 


• 











5* "5 

HI 


1 

1 
1 

j 


j 


j 

i 


t 


■ i 


i 
j 


: 


i 

j 

t 

I 




r 

1 

1 


1 
1 




1 


.J i 


[ 




1 

i 


500 
VOO 




\ 


i 




/ j 






; I 


i 
\ 

\ 

i 

i 


{ 

i 
] 

! 

1 

i 


I 

i 

1 

1 


1 


i I 

i : 

": i 
■■ 1 

\ \ 


i 

1 

1 

j 




1 
1 

i 

i 

I 
1 


\ 


i 




j 


J { 


1 
1 




i 




1 
r 

\ 

J 


I 
! 

j 


\ 

i 


\ 
1 


1 ,* 

| \ 


i 




! 
1 

1 
\ 






\ 


1 


i 


1 

| 


( i 


: 




\ 




300 


t 


, 


•>. E\ 


-,£- r 


-£• | : 


j 




| 






>— VT 






5 ' : 


1 










Cm 

\ 
\ 
\ 




\ 


i 

--S v 


\ \ 
-s \ I 

\ 1 
i ■ 


} 
f 




i 

\ 

1 
j 

i 
i 


I 


ZOO 
/OO 


1 
I 
\ 


I 


-B 




i 1 


1 

i 




i 

1 




1 

\—JB 


\ "3 


i 


\ \ 
i. 5 


: 

\ \ 




i ! 
'; "l 

\ \ 






















Time 


Jnferisa/ /r? 


■seconds 






/s zo zo 


20 /s 

J_l 


• zo /s /s 

1 1 1 


zo zo 

1 1 


1 


/S /S ZO 

1 1 1 



Fig. 2. — Inverse-rate heating curves of hardened steel, showing effect of previous tempering 

for jo minutes on Act 



542 



Scientific Papers of the Bureau of Standards 



Wot. r« 



temperatures of the principal curve bends caused by the Ac t trans- 
formation were taken as denoted on the curves by B, M, and E, 
beginning, maximum, and end, respectively. The rate of heating 
given is that just before the beginning of the transformation. The 
values in the column of Table 2 marked "Intensity" represent 



SOO 



WO 



3O0 



2O0 



/oo 



0.95 Percent C Steel 
Oaenchec/ /r> o/7 from <SOO"C 
~femperea e?t ^00°C Tempered at £JO°C 
5 mm 30 min. 60 m//?. 5 min. 30min. bOmin. 



'■■: £ 



\-Af 



%u 



m V 



« 



IB 






V* 



«V> 






Time interna/ in secone/s 
/s so /s so /s so /s so /s so /s so 



Ftg. 3. — Inverse-rate of heating curves on hardened steels, showing effect of duration of 

previous tempering on Act 

the difference in seconds between the time at the maximum and 
at the end of Ac t , except in the case of the austenitic steel, where 
they represent temperature drop. 

The temperature of the maximum of A^ (maximum temperature 
before decalescence when that phenomenon was observed) is also 



ScoU "1 
Mtrvius} 



Thermal Changes of Hardened Steels 



543 







Carbon Steels 








Quenched from above /?c a 




°c 


C .03 
Mn - 
Si .0/ 


MO Mb MU .73 .95 I.OI 

— .35 1.00 .38 .ZZ — 

.01 .Ob .OZ .01 .21 .01 




4,00 












|| 11-11 

i 5 ! 










Mi! 


i 






. 




'( I I ! 


i 
j 




SOO 






i ! ! . f 


t 








1 j i • 1 








, 




III' 


l i 










•! } i i '■ 






1 




i ! 1 

• ! - s \ \ \ \ 




¥00 


t 


'■1 I'll 






, 


\ \ \ \ \ 

i ! i ■: •. 
i i \ 




300 




! 


\ \ 111 

! i i '. i 

| '. \ -, \ 


i 


t 
1 

s 


)-£ )~ •-" .-••'"/••■""" 


;.•.— 




! 


{-M \ \ \ \ e 






i 

\ 


\ \ \ \ \ \ 
\ V ■ \ \ \ 

■■■ \ \ \ \ 


i 


ZOO 
JOO 


\ 


\ ; i \ i 






11 \ \ \ 

i i_ ■:_ '— \ 

\ \ \ ■••. \ \ 


I 

i 
i 












7//77e interval in seconds 






/5 ZO ZO ZO ZS ZO ZO £0 

II 1 1 1 1 1 1 1 1 1 1 1 II 


es 30 

1 1 



Fig. 4. — Inverse-rate heating curves of quenched steel, showing effect of composition 
4218°— 20 2 



544 Scientific Papers of the Bureau of Standards Ivoi. 16 

given, but the heating curves are not plotted to show Aci, in order 
to avoid excessive reduction of the curves on reproduction. 

1. EFFECT OF RATE OF HEATING 

Rate of heating has a considerable effect on the temperature 
and form of Ac t for the comparatively fast rates required by ther- 



"C 

WO 



500 



WO 



300 



£00 



too 





r^u^tenific 
Quenched in wafer 


Iron- 
from 


Carbon /7//oy 

;/oo°c S ame *W ed 

in liquid air 


\ \ 
t i 

> ;■ 
\ \ 


> 

K 
i 

i 




J © 

if. . 

I. 




: © 




® 


1 © 

i 




I, 

> 

V 

\ 


— At 

\~M 1 J/tf' 


-"-=-^ 






— >-• • 130" 


-frj/r 








s 


-zrj° 




£85° ' 

\ 




1 






\--B 
:■ \ 






1 


Time 

15 20 25 JO Ji 

1 1 1 1 1 1 1 


interval in s 

5 /O /5 ZO 45 

Mill 


econds 

5 10 
1 1 1 


IS zo 

1 1 1 


IS 20 25 30 OS 

MINI 



Fig. 5. — Inverse-rate heating curves of austeniiic iron-carbon alloy {1.Q4 P er cent O 

mal analysis, as may be seen from the curves of Fig. i , which were 
taken on the 0.95 per cent C steel. The principal data taken from 
these curves are plotted with rate of heating as the abscissas, in 
Fig. 6. It will be noted from this figure that the temperature 
characteristics of Ac t for zero rate are 155, 250, and 260 C, respec- 
tively, for the beginning, maximum, and end of the transforma- 
tion. This appears to represent the progress of the transformation 



Scott 1 
Mvvius} 



Thermal Changes of Hardened Steels 



545 



for a tempering time approximating normal tempering conditions, 
probably about 30 minutes. From the sharpness of the begin- 
ning of the transformation it would appear that the quenched steel 
is the equivalent of a steel instantaneously cooled and then drawn 
in the neighborhood of 150 C. Fig. 6 illustrates this interesting 
point: That for the size of specimen used in this case there is no 
appreciable difference between the thermal characteristics of an 
oil-quenched and of a water-quenched specimen. 

j 
2. EFFECT OF TEMPERING TEMPERATURE 

The heating curves represent the progress of tempering for a 
necessarily very short time at any temperature in the Ac t range. 
To show the effect of hold- 
ing for a definite time at 
several tempering temper- 
atures on the characteris- 
tics of Ac t , heating curves 
were taken on specimens 
of the 0.95 per cent C steel 
quenched in oil from 8oo° 
C and tempered 30 min- 
utes at the temperatures 
given in Fig. 2 and Table 2. 

From a consideration of 
these data it may be seen: 
(1) That the beginning of 
Ac t is from 10 to 17 C 
higher than the tempering 
temperature when that is 
above 200 C ; (2) that the 
transformation is com- 
pleted at a temperature 
between 250 and 270 C; 
and (3) that for each tem- 
perature up to 250 C there 
is a definite and character- 
istic form of curve. The 
estimated temperature of the end of Ac t for zero rate (260 C) is, 
therefore, from (2) in practical agreement with the end of the 
transformatior for a tempering period of 30 minutes. From (1) 
and (3) it is evident that the heating curves might be used to esti- 
mate the previous tempering temperature within certain limits. 




Rate of heating 



.30 X/sec. 



Fig. 6. — Effect of rate of heating on temperature 
and intensity of heat evolution of 0.95 per cent 
C steel 



546 Scientific Papers of the Bureau of Standards ivoi. 16 

It may be of interest to note that, for a tempering temperature 
of 270 and 300 C (Fig. 2) , there is a slight deflection of the curves 
to the right, indicating an absorption of heat over the range of 
about 350 to 450 C, which is in conformity with the observations 
of Heyn and Bauer 10 under similar conditions. 

3. EFFECT OF TIME AT TEMPERING TEMPERATURE 

It has long been recognized that the time of holding at a tem- 
pering temperature has a very considerable effect on the resulting 
physical properties, and it is even held that a long time at a low 
temperature is equivalent to a short time at a higher temperature. 
The thermal curves of steels tempered for different lengths of time 
in the Ac, range should, therefore, throw some light on the validity 
of this much discussed proposition. 

In Fig. 3 heating curves are given to show the effect of main- 
taining a steel for different lengths of time at the tempering tem- 
perature. The 0.95 per cent C steel, hardened by being quenched 
in oil from 8oo° C, was used. Specimens were maintained for 5, 
30, and 60 minutes at each of the two tempering temperatures, 
200 and 230 C, chosen because they represent temperatures at 
which tempering is well in progress, but not to such an extent as 
to eliminate the thermal effect. 

It may be noted from these curves and the compiled data of 
Table 2: (1) That the beginning of Ac t is higher for a long than 
for a short exposure at the tempering temperature; (2) that the 
intensity of the transformation is less for a long tempering period 
than for a shorter one; and (3) that the rate of progress of the 
transformation is greater at the higher tempering temperature 
than at the lower one. 

From (1) and ^(2) it is apparent that time has a decided effect 
on the transformation characteristics. The third conclusion is 
evident from the fact that at 200 C an exposure of 60 minutes is 
necessary to reduce markedly the intensity of Ac t , while at a tem- 
perature only 30 C higher the intensity is much more strongly 
reduced by a 30-minute exposure. This is in agreement with the 
tempering experiments of Barus and Strouhal, 11 whose measure- 
ments of electrical resistance and thermal emf show the rate of 
transformation to be much greater at the higher tempering tem- 
peratures in the Ac t range. This indicates further that the tem- 
pering time of 30 minutes used in the preceding section represents 
actual equilibrium or zero-rate conditions at the temperature of 
the end of Ac t , though, of course, not at lower temperatures. 

10 See footnote 3. "Barus and Strouhal. Bull. U. S. Geological Survey. No. 14; 1885. 



Scott "I 
Moviusi 



Thermal Changes of Hardened Steels 
TABLE 2. — Thermal Characteristics of Hardened Carbon Steels 



547 



Composition 



Mn 



Si 



Heat treatment 



Quench- 
ing 
temper- 
ature 



Quench' 
ing 
me- 
dium 



Tem- 
pering 
tem- 
pera- 
ture 



Time 

at 
tem- 
pering 
tem- 
pera- 
ture 



Rate 
of 

heat 
ing 



Act temperature 



Be- 
gin- 
ning 



Maxi- 
mum 



End 



Intensity 
ot heat 
evolution 



Aci 
max- 
imum 



Per 
cent 



Per 

cent 



Per 
cent 



.95 



.24 



.40 
.46 
.44 
.73 
.95 
1.01 



.35 
1.00 
.38 
.22 



.01 
.06 
.02 
.01 
.24 
.01 



800 
800 
800 

800 



(") 



800 
800 
800 
800 
800 
800 
800 
800 



800 
800 
800 
800 
800 
800 



900 
900 
900 
900 



1,100 
1,100 
1,100 
1,100 



Water. 
..do... 



..do. 
Oil... 



Oil... 

..do.. 
..do. 
..do. 
..do. 
..do.. 
..do. 
..do. 
..do.. 



Oil... 
..do. 
..do. 
..do. 
..do. 
..do.. 



Water. 
..do... 
Oil-'... 

Water. 
Oil.... 
Water. 



Water. 
..do... 
..do... 
..do... 



200 
230 
250 
270 
300 
350 
400 



200 
200 
200 
230 
230 
230 



Min- 
utes 



°C 

/sec. 



Seconds 



(1) Effect of heating at different rates 



0.23 


183 


290 


319 


.16 


178 


285 


308 


.05 


162 


261 


282 


.22 


167 


273 


295 


.23 

















7 
14 
8.5 




733 
736 



(2) Effect of tempering at different temperatures 



0.12 


167 


273 


295 


.13 


215 


275 


300 


.13 


240 


282 


307 


.15 


267 




310 


.16 








.15 








.12 








.17 








. 15 

















8.5 

9.0 

1. 5 

0.5 













733 
733 
733 
734 
734 
733 
733 
733 
735 



(3) Effect of tempering for different periods of 
time 



0.15 


204 


276 


300 


.13 


215 


275 


300 


.16 


224 


273 


300 


.14 


224 


271 


296 


.13 


240 


282 


307 


.13 


244 


289 


350 



9 

9 

4 

5.5 

1.5 

0.5 



(4) Results for different compositions 



0.12 


167 


259 


278 


.12 


174 


270 


286 


.13 


174 


276 


297 


.12 


175 


271 


292 


.12 


167 


273 


295 


.10 


167 


261 


284 



731 
731 
727 
731 
733 
728 



(5) Results for austenitic structure 



.08 


174 


308 


319 


.11 


182 


347 


361 


.15 


184 


353 


366 


.12 


179 


302 


317 



' C Temp, 
drop 

2 

24 



731 



a Annealed. 



6 Air cooled. 



548 



Scientific Papers of the Bureau of Standards 



{Vol. 16 



It may be inferred from the nearly identical characteristics of 
Ac t following tempering for 60 minutes at 200 C and 5 minutes at 
a temperature 30 C higher, that these two treatments produce 
the same structural condition, but, because the rate of transforma- 
tion changes with temperature, it does not follow that this par- 
ticular relation holds quantitatively for any other temperatures in 
the Ac t range. 

In general, however, the effect of time may be regarded as 

equivalent to that of temperature within limits as far as the 

characteristics of Ac t are a criterion of the constitutional changes 

in the steel. 

4. EFFECT OF COMPOSITION 

For the sake of comparison of the several martensitic steels 
investigated, the temperature values of Ac t taken from Table 2 
have been given a small correction on the basis of Fig. 6 to reduce 
them to a constant rate of heating of o.io°C per second; these 
values are given in Table 3. By comparing the synthetic steels 
in the first group, low in manganese, or the commercial steels in 
the second group, containing 0.20 to 0.40 per cent manganese, 
with respect to the variable carbon, one may see that the maxi- 
mum and end of Ac t are somewhat higher for the higher carbon 
contents and that the transformation intensity is approximately 
proportional to the carbon content. 

TABLE 3. — Transformation Characteristics of Martensitic Steels for Rate of Heating 

of 0.10° C per Second 



SYNTHETIC STEELS 


C 


Mn 


SI 


Ac, temperature 


Intensity 


Beginning 


Maximum 


End 


Per cent 

0.40 

1.01 

<H.94 


Per cent 


Per cent 

0.01 

.01 

.01 


°c 

165 

167 
177 


°C 

255 
261 
298 


°C 

274 
284 
313 


Seconds 

2 

13 

32 








COMMERCIAL STEELS 


.46 
.73 
.95 


.35 
.38 
.22 


.06 
.01 
.24 


172 
173 
165 


266 
267 
269 


282 
288 
291 


3 

7 
8.5 




S' 


fNTHETIC 1 PER CENT MN ST 


EEL 




.44 


1.00 


.02 170 270 


291 


6 



a Martensitic by immersion in liquid air; curve 4, Fig. 5. 



jVfori,„] Thermal Changes of Hardened Steels 549 

The effect of an increase of carbon on the heat evolution is to 
augment correspondingly the rate at a given stage in the progress 
of the transformation for a given furnace rate. This increase in 
rate from the effect noted in Section III— 1 , on rate of heating, will 
raise the temperature of the maximum and end of Ac t . The in- 
crease in temperature of the maximum and end of Ac t with in- 
creasing carbon, being small, is probably due entirely to the 
augmented rate of heating. This factor being ineffective for very 
slow or zero rate of heating, it may be stated that, for this case, 
the carbon does not materially affect the maximum and end of 
Ac t . Likewise, the rate at the beginning is unaffected by the 
subsequent heat evolution, so that the constancy of that point 
verifies the conclusion that the temperature of Ac t is practically 
independent of carbon content under conditions which render the 
effect of intensity impotent. 

From Table 3 it may be of interest to note further that Ac t is 
slightly higher in the commercial than in the pure synthetic steels 
of the same carbon content, and that the characteristics of Ac t 
for the steel containing 0.73 per cent carbon and 0.38 per cent 
manganese are practically identical with those for the steel con- 
taining 0.44 per cent carbon and 1.0 per cent manganese. 

S. EFFECT OF AUSTENITIC STRUCTURE 

For obtaining the data presented in preceding sections, the 
material used was a martensitic steel, but it was believed to be 
of some interest to study also the thermal changes accompanying 
the decomposition of an austenitic matrix in a carbon steel. 
With this in mind, a steel of 1.94 per cent C content was quenched 
in water, the resulting structure being uniformly austenitic 
(Fig. 7 a). 

The curves taken on specimens given such treatment are 
shown in Fig. 5, curves /, 2, and 3, and the data taken from 
them, are shown in Table 2. The transformation observed was 
very intense, and in one case (Fig. 5, curve 3) the heating ad- 
vanced so rapidly that the operator could not follow it. In 
every case the heat evolution increased the specimen temperature 
at the end of the transformation so that it exceeded the normal 
furnace temperature, and a drop of temperature was then re- 
corded. This amounted to 2° C in curve 1 and 24 C in curve 2; 
the drop for curve 3, though not recorded, was very considerable. 

In these austenitic steels the effect of rate of heating upon the 
temperature of Ac t is much more pronounced than in the marten- 



550 Scientific Papers of the Bureau of Standards ivot. 16 

sitic steels, though with slow rates the difference between the 
two types of steel is small, if any. Thus by comparison of 
curve i for the austenitic steel (rate of heating, 0.08 C per 
second) with the curve for the 0.95 per cent C steel (rate of 
heating, 0.16 C per second), in both of which cases the rate is 
approximately the same at the maximum, one may note that 
the maxmum for the austenitic steel is 23 C higher and the end 
only ii° C higher than for the martensitic steel, even though an 
actual temperature drop occurred in the former case. This 
would indicate that for very slow rates there would be little 
temperature difference between Ac t for a martensitic structure 
and Ac t for an austenitic one. Here again the fact that the 
beginning for both is practically the same verifies the conclusion 
that the heat evolution of the transformation materially affects 
its observed temperature for sensible rates of heating. 

Curve 4 of Fig. 5 shows the thermal characteristics of one of 
the austenitic steels after exposure for 30 minutes in liquid air. 
This treatment rendered the steel partially martensitic in struc- 
ture (Fig. 76). Comparison of the heating curve 4 for this 
sample with curve 2, the rate of heating being essentially the 
same in both cases, shows that treatment in liquid air causes (1) 
a lowering of the maximum and end of the transformation by 
about 45 C, and (2) an evolution of heat much less intense, 
without any recorded drop of temperature. This indicates a 
marked structural change, evidently from austenite to martensite, 
on immersion of the austenitic steel in liquid air. 

Further examination of the heating curves of the austenitic 
steels reveals another significant phenomenon. An inflection in 
these curves may be noted at 273 C for curve 2, 285 C for curve 
3, and somewhat lower for curve 1, at which temperature there is 
an augmentation of the rate of heating. It is evident from 
curves 2 and 4 that the heat evolution of the latter (marten- 
sitic) steel starts to drop off, while the former (austenitic) steel 
is considerably intensified just above the inflection temperature 
of 2 73 C. This indicates two stages in the decomposition of the 
austenitic steel — namely, the low-temperature stage, probably a 
manifestation of the simple carbide precipitation, and the high- 
temperature stage, which is the same intensified by the A 3 and 
A, transformation. In the case of the martensitic steel the 
change designated by Ac t is very probably due only to the car- 
bide precipitation. 



Scientific Papers of the Bureau of Standards, Vol. 16 




to 




to 
Fig. 7. — Microstructure of 1.94 per cent C steel quenched in water from 
1100° C. Etched with 2 per cent alcoholic HN0 3 

(a) As quenched. X500 

(b) Same as (a), but dipped in liquid air for 30 minutes. X2co 



M° "ms] Thermal Changes of Hardened Steels 551 

IV. RELATION OF CHANGES IN PHYSICAL PROPERTIES TO 

HEAT EVOLUTION 

The thermal curves presented here show a rapidly increasing 
heat evolution (Ac r ) from 155 C to 250 C (ending abruptly at 
about 260 C) for a very slow heating rate. This last tempera- 
ture, 260 C, very probably represents the completion of the 
change from martensite into troostite. A consideration of the 
changes in physical properties on tempering quenched steels 
through the Ac t range should, therefore, assist materially in 
determining other characteristics of this change. 

1. MARTENSITIC STEEL 

In order to make a reliable comparison of the characteristics 
of Ac t with the changes in the standard scleroscope and Brinell 
hardness numbers, measurements were made on samples of the 
0.95 per cent C steel used for the majority of the thermal curves. 
The results are shown in Figs. 8 and 9, the recording scleroscope 
being used in the first case and the usual Brinell equipment in the 
latter. A fresh surface of the ball was taken for every impression. 12 

For the purpose of comparing some of the other physical proper- 
ties with the heat evolution, Fig. 10 was prepared. Curves 1 
and 2 of this figure were obtained from the magnetic data of 
Burrows and Fahy 13 and are expressed in gausses per square 
centimeter; curves 3 and 4 were obtained from the electric re- 
sistance and thermal emf data of Campbell, 14 and curve 5 was 
obtained from the density values of Schulz. 15 In all cases marten- 
sitic carbon steels were used of approximately the same carbon 
content as the 0.95 per cent C steel used here. 

Upon returning to the hardness curves of Figs. 8 and 9 one 
may see that the scleroscope hardness does not drop off abruptly 
until slightly above the temperature (260 C) of the end of Ac t , 
and that the Brinell hardness begins to drop linearly imme- 
diately above the beginning of Ac t . Thus there exists in both the 
hardness curves an inflection closely related to fundamental tem- 
perature characteristics of the heat evolution. 

Consideration next of curves 1 and 2, Fig. 10, for coercive force 
and maximum induction, respectively, will show that these 

12 Since these Brinell data were obtained, the work of Chevenard (see footnote 5) appeared, containing a 
curve for an 0.85 per cent C steel practically identical in form with Fig. 9, with the exception that his values 
are somewhat lower. 

13 Burrows and Fahy. Trans. A. S. T. M., 19, part II, p. 5; 1919. 
» Campbell, J. Iron and Steel Inst., 94, p. 36S; 1916. 

15 Schulz, Forschungsarbeiten, No. 161, p. 1; I9r4. 



552 



Scientific Papers of the Bureau of Standards 



IVol. 16 



properties change with an increasing rate over the range 20 to 
300 C in the same manner as the heat evolution. The tempering 
temperature steps were not taken sufficiently close to define the 
end point of this change, but it coincides substantially with the 
end of Ac t . The curves 3 and 4, for thermal emf against pure iron 
and specific resistance, respectively, are practically parallel and 
may, therefore, be considered as a unit. It may be observed 
too 



90 



80 



70 



5 

! 

* 

v. so 



vo 



JO 





— ■ ( 


> 


CarSd/7 Too/ S/ee/ 

Q.95; /Vs?,.ZZ;S/,.ZV 






" 


~°*% 




Quenc/ied m water 
from 300 "C 


























ov. 










Sa/7 
• Sam 


i/b/e r7 " 

b/e £ . 


\/i>yZby 












/6"> 

























/00 ZOO 300 WO 500 

7~err7fier/ny temperature 



(,00 



700 



FlG. 8. — Effect of tempering temperature on scleroscope hardness of o.Q$ per cent C steel 

that the change is about 85 per cent complete at 300 C, or in the 
Ac t range. The density curve 5 is somewhat irregular, but the 
maximum rate of change occurs in the vicinity of the end of Ac<. 
From the foregoing analysis it is evident that the changes in 
the physical properties considered are related very closely to the 
heat evolution Ac t , particularly in the case of the magnetic proper- 
ties, maximum induction, and coercive force. These relations 
are forceful indications of a natural boundary between martensite 
and the troostite produced at about 260 C on tempering. Such 
a boundary should be detectable also by the changes in micro- 



Scott "I 
Movnts} 



Thermal Changes of Hardened Steels 



553 



structure. Authorities, however, differ on the temperature of 
this boundary for simple steels, and place it anywhere in the 
range from 250 C to 400° C. Careful observers have studied 
this change, and while not suggesting an end point, have made 
observations indicative of one in the region of the end of Ac t . 

Howe and Levy, 16 after quenching eutectoid carbon steel from 
1100 C to water, find that on a 5-minute exposure to 300 C 



700 



600 

\ 
1 

*soo 

<o 
to 
&VOO 

\ 

^JOO 
($£00 



/OO 











Carbon Too/ Sfee/ 




u— 


■c< 


C..95; Mn,.22; 3/..2V 
J3or J6y'/ z by6/n. 




( 


!\ 












oV> 
















X 










Quenched in wafer 
from <900 " C 











































/OO 



200 J00 400 500 600 700 "C 
Tempering temperature 

Fig. 9. — Effect of tempering temperature on Brinell hardness of 0.Q5 per cent C steel 

the original white martensite needles are almost completely 
broken up. Heyn 17 notes a coarsening of the needle structure at 
275° C. 

These observations are indicative of a structural change in the 
vicinity of the end of the heat evolution. The region under 
investigation is thus narrowed down, and future observers should 
have little difficulty in denning precisely the nature of the accom- 
panying changes. 



16 Howe and Levy, Trans. A. S. T. M.. 16, part II, p. 7; 1916. 

" Heyn quoted by Sauveur, The Metallography and Heat Treatment of Steel, p. 304. 



554 



Scientific Papers of the Bureau of Standards 



IVol. 16 



2. AUSTENITIC STEEL 



In a foregoing section (p. 550) attention was called to a sharp 
change in direction of the heating curves of the austenitic steel. 
This inflection, denoting an abrupt increase in the rate of heat 
evolution, was noted to start at about the temperature at which 




/00 200 J00 VOO 500 

Te/nfoer/na temperature 



too 



700 'C 



Fig. 10. — Change of physical properties with tempering tem- 
perature of martensitic carbon steels 

the heat evolution in a martensitic steel begins to disappear. It 
may therefore be of some interest to compare this thermal 
behavior of the steel with the density changes in similar steel. 

In Fig. 1 1 are plotted density values given by Maurer 18 for a 
1.66 per cent C steel quenched in water from 1,050° C. The 
resulting structure is not completely austenitic, but nearly so. 

u See footnote 2. 



Scott "| 
Moviusl 



Thermal Changes 0} Hardened Steels 



555 



The curve shows three distinct regions: (1) 20 to 150 C, in 
which the density increases as in a martensitic steel; (2) 150 to 
250 C, in which a drop occurs which recalls the second stage of 
the heat change of the austenitic steel; and (3) above 250 C, in 
which it follows the normal course of a martensitic steel. Since 
martensitization implies a decrease in density (see the black 
circles of Fig. 1 1 , representing the density change on immersion 
in liquid air), the second step with a density drop is evidently 
attributable to completion of the change from austenite to mar- 
tensite, which is more or less transformed into troostite. The 



























G/.66; A/r?,.09; S/, ./O 

Quenc/iet? from /050"C 
(flanrer) 













'cm 3 
V.75 



\7.70 



7.05 



-ZOO +200 VOO 600 800 'C 

Te/nfoer/hg femjberafure 

Fig. 11. — Change in density with tempering temperature of 
semiaustenitic carbon steel (Maurer) 

augmentation of the heat evolution of the austenitic steels, as 
previously explained, is therefore definitely verified. 

V. SUMMARY 

The transformation, observed as an evolution of heat, on 
heating curves of hardened steel has been designated here as Ac t , 
and its characteristics as revealed in carbon steels have been 
investigated. The effect of several variables was noted with the 
following conclusions: 

1. An increase in the rate of heating raises markedly the tem- 
perature of Ac t for a 0.95 per cent C martensitic steel and has a 
yet more marked effect for an austenitic carbon steel. For zero 
rate of heating there appears, however, to be little, if any, difference 
between the principal temperatures, whether the steel is of high 
or low carbon content or whether it is martensitic or austenitic. 
The principal temperatures for the 0.95 per cent C martensitic 



556 Scientific Papers of the Bureau of Standards Wot. 16 

steel were found to be 155, 250, and 260 C, respectively, for the 
beginning, maximum, and end. 

2. The results obtained for specimens tempered at different 
temperatures before taking heating curves confirm substantially 
the temperature of the end of Ac t just given. 

3. Tempering for a short time at a temperature within the Ac t 
range has an effect on the transformation characteristics similar 
to tempering for a longer time at a somewhat lower temperature. 

4. The heat evolution of the austenitic steel takes place in two 
steps, the second being probably connected with the transition 
from austenite to martensite. 

5. A survey of the changes in some physical properties of mar- 
tensitic carbon steels through the tempering range leads to the 
conclusion that these changes are all directly related to the heat 
evolution observed, but only in the case of the magnetic proper- 
ties, coercive force, and maximum induction is the change of the 
same type. 

6. The change in density of a semiaustenitic carbon steel pro- 
ceeds in steps similar to the heat evolution of the austenitic steel. 

7. The changes in microstructure on tempering martensitic 
steels are unquestionably related to the heat evolution, but 
further study is necessary to establish fully this relation. The end 
point (260 C for zero rate) of Ac t may very properly be taken as 
the natural boundary between martensite and the troostite of 
tempering, representing as it does the end of the transformation 
suppressed on rapid cooling. 

The competent assistance of H. A. Wadsworth has greatly 
facilitated this investigation. 
Washington, April 22, 1920. 



LIBRARY OF CONGRESS 



003 129 912 3 



